The invention relates to switching power supply (SPS) receiving AC line voltage or DC voltage, particularly for power supply systems requiring high output power, high efficiency and fixed or variable output voltage. Peak currents of switches are smallest possible as to accomplish best possible line and component utilizations. In particular, inrush and surge currents are completely eliminated.
Conventional SPSs convert AC line voltage into DC voltage. In particular, the AC voltage is rectified and further applied to an input capacitor. Charging the capacitor causes inrush current and surge currents every peak of the AC voltage. Pulse width modulation (PWM) is used which results in numerous problems including high circuit complexity, stability problems and significantly higher effective switching frequency. Numerous interference suppressors and protection circuits are inevitable. Power factor correction circuits are employed which further increase complexity and decrease efficiency.
Power factor correction circuits employ a boost switching circuit. Rectified line voltage is applied thereto. Therefore, switching frequency or PWM varies in a large range. The circuit is practically inoperative near line voltage crossover. The voltage provided by the boost circuit is inherently greater than the peak of the line voltage. A troublesome mechanical switch for reducing capacitor voltages is employed. Moreover, an output capacitor is coupled to ground, whereby the inrush current is inevitable.
Power factor defines performance of an SPS. It is a ratio of the SPS output power over input power. The input power is RMS input voltage multiplied by RMS input current. The power factor is often specified only for the correction circuit. In fact, the power factor depends on efficiency of the entire SPS. A conventional power factor correction circuit simulates sinusoidal input current which is in phase with the line voltage. Therefore, the SPS simulates a resistive load. However, the power factor correction circuit must sustain peak currents at least 1.41 times higher than the RMS input current. The respective control circuit is complicated, wherein many compromises are inevitable.